In 1996, Ukraine sent the last of its Soviet-era nukes off to Russia, completing an agreement that had been negotiated following the breakup of the Soviet Union and the establishment of Ukraine as an independent country.
How much different would the present crisis be if that had never happened?
Which brings us to the logical conclusion for any country in the wake of the current crisis: If you give up your weapons of mass destruction, you're liable to be bullied by any neighbor, declining world power or not, that can mass troops on or near your border. Iran appeared to draw that conclusion based on America's invasion of Iraq; now it's something no nation can afford to ignore. If you think the spread of nukes is a bad thing, the implications in eastern Europe and Asia ought to make you lose a lot of sleep for a good long time. No matter what happens with today "vote" in Crimea - and no matter what the West's ultimate response is - Russia's actions will affect geopolitics for a generation at least.
Paul Berman recently put the Ukraine into historical perspective in an article in the New Republic, with a (somewhat) optimistic spin:
We do seem to be on the brink of Cold War II, which might end up being a long affair. We ought to recall that Cold War I was, despite its reputation, not really a stable era. Russia in its Soviet and post-Soviet incarnations has never succeeded in establishing a zone of tranquility, except for relatively brief periods. The entire concept of Russian domination has proved to be a formula for repeated revolutions. The revolutions of the past took place in 1953, 1956, 1968, 1989, 2000, 2003, 2004, and 2014 (in, respectively, East Germany, Poland and Hungary, Czechoslovakia, everywhere, Serbia, Georgia, Ukraine, and Ukraine). These events suggest a pattern, though, and maybe the pattern should be encouraged. The pattern is a generally eastward drift, and the purpose in encouraging it ought to be what used to be called, in language I never liked, regime change—achieved not of course by military adventures of our own but by the citizens of Moscow and St. Petersburg, aided by whatever peaceful support we can provide.